<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>半圆角</title>
    <style type="text/css">
      .box {
        width: 100vw;
        height: 100vh;
        background-color: aqua;
      }

      .tool {
        width: 30px;
        height: 50px;
        border: 1px solid black;
        background-color: rgb(92, 79, 105);
        border-radius: 100px 0 0 100px;
        position: absolute;
        right: 0px;
        top: 50vh;
        border: none;
        outline: none;
        z-index: 100;
      }

      .tool:hover {
        background-color: black;
      }
    </style>
  </head>

  <body>
    <div class="box">
      <button class="tool"></button>
    </div>
  </body>
</html>
